There is now a plethora of neurochemically distinct populations of primary afferent fibers that respond selectively to different pain and itch-producing stimuli. These nociceptive and pruritoceptive afferent fibers, in turn, engage spinal cord circuits that transmit these messages to higher brain centers. Determining to what extent these messages are transmitted along segregated (i.e., labeled lines) or convergent CNS pathways is a major objective of the studies outlined in this proposal. Using a multidisciplinary approach that builds upon our development of powerful genetic and virus-based neuroanatomical tracing methodologies, our studies will provide a comprehensive anatomical and functional analysis of the spinal cord circuits that contribute to pain and itch and will addres specifically the extent of convergence of these complex somatosensory modalities. In Specific Aim 1, we will use Cre-dependent, anterograde and retrograde transneuronal viral vectors to study the direct and indirect circuits that influence and are engaged by major classes of nociceptive and pruritoceptive primary afferent and spinal cord neurons, including the neurokinin-1 receptor-expressing subset of projection neurons, and four major, and distinct interneuron populations of the superficial dorsal horn, namely those expressing the gamma isoform of protein kinase C, gastrin-releasing peptide, dynorphin and the alpha subtype of estrogen receptor. In Specific Aim 2, we will use the novel transgenic TRAP mouse to provide functional correlates of the networks engaged by afferents that respond to different stimulus modalities. Finally in Specific Aim 3, we will turn our attention to the much less studied large diameter primary afferents, which are significant contributors to mechanical allodynia in the setting of injury. We will use an innovative method not only to characterize selectively the circuits engaged by these afferents, but also the powerful RiboTag method to identify distinct molecular markers of this subset of primary afferents. Taken together, our proposed studies will provide a very in depth analysis of the spinal cord circuits that transmit nociceptive and pruritoceptive messages and that underlie the pathological pain and itch conditions that occur in the setting of injury.